First, you should split your essay into two parts. These should be the elements of news as one section and the sources of news as the other. A good source for things like this is Lance Bennett's News: The Politics of Illusion.

Within elements of news, you might want to talk about the inherent bias of the news in favor of reporting things that are easily understood and that have a good story line. For example, political news tends to focus on who won and who lost in various incidents (like the current discussion over the debt ceiling) rather than on the actual merits of the various proposals. This is because people understand conflict and the idea of who won and who lost makes for a good story with a beginning, middle, and end.

As far as sources, one of the most important facts about this is that journalists tend to rely on official sources. These are the most (allegedly) credible source of news and journalists need access to officials so that they can get news. This reliance on official sources leads to a bias in favor of official versions of the news.